Bendable mirrors and method of manufacture

ABSTRACT

A heat formable mirror is formed by sputter depositing upon a sheet such as glass a reflective coating utilizing a base layer of silicon or a combination of silicon and stainless steel films, a reflective layer formed of a reflective metallic film such as niobium, titanium or aluminum, and a protective layer comprising preferably silicon nitride. The resulting mirror can be heat formed at elevated temperatures to form a curved mirror having a reflective coating free of objectionable defects.

This application is a continuation-in-part of U.S. application Ser. No.08/496,906 filed Jun. 29, 1995.

FIELD OF THE INVENTION

The present invention relates to mirrors, and more particularly tomirrors that are formed utilizing flat substrates which subsequently areheat-bent into a desired curve configuration.

BACKGROUND OF THE INVENTION

Curved mirrors commonly are employed as rearview mirrors for motorvehicles, as reflecting surfaces for telescopes, and the like. Curvedmirrors commonly are formed by first forming a glass sheet or othersheet-like substrate into the desired curved configuration, andsubsequently applying a reflective coating to one side or the other ofthe substrate. For example, curved mirrors of the type used in carnivalsto provide amusing, contorted reflections of a viewer may be made byfirst forming a sheet of glass into the desired shape, and then coatingone surface of the glass with metallic silver and a protective paintovercoat.

Mirrors also can be manufactured by employing a magnetron sputteringtechnique such as that described in Chapin, U.S. Pat. No. 4,166,018.Mirrors of this type may use chromium or silver as the reflective layer.When curved mirrors are manufactured using a magnetron sputteringprocess, the glass substrates for the mirrors are first bent as desiredtypically in a size that would produce two or more mirrors. After thebent glass pieces are washed in a curved glass batch-type washer or on acarrier in an on-line washing system, they are placed on an appropriatecarrier and are coated by magnetron sputtering. Due to the curvature ofthe or aluminum, niobium being preferred. The third layer is aprotective film that is positioned further from the substrate than thereflective layer, the protective film providing sufficient oxygenpermeation inhibition as to prevent the reflectance of the mirror fromdecreasing to less than 50% upon heat bending. The third layerpreferably comprises sputter-deposited silicon nitride,sputter-deposited aluminum oxide or sputter-deposited silicon dioxide;of these, silicon nitride is preferred.

When a heat-formable mirror of the invention is heat formed at atemperature above the temperature at which the layers of the reflectivecoating are deposited, atomic diffusion and/or structural rearrangementscan occur between the various sputtered films, changing the reflectiveproperties of the bent mirror product. The heat formable mirrors of theinvention, however, largely and preferably fully retain their importantmirror optical properties (low transmissivity, high reflectance) whensubjected to heating and bending in this manner.

Thus, in another embodiment, the invention relates to a curved mirrorthat is produced by providing a heat-formable mirror of the typedescribed above, and subjecting the mirror to a temperature at which thesubstrate is capable of plastic deformation (e.g., the glass transitiontemperature in the case of glass substrates), bending the flat mirror atthat temperature into a desired curved conformation to produce a curvedmirror, and then cooling the curved mirror while maintaining its curvedconformation. The resulting curved mirror desirably retains at leastabout 100% of the reflectance and not over about 150% of thetransmissivity of the heat-formable flat mirror from which it was made.

Curved mirrors of the invention desirably display a hemisphericalreflectance (as measured using a reflectometer and integrating sphereover the wavelength range of 200 to 2600 nm) of at least 50% and atransmissivity not greater than about 4.0%. "Reflectance" herein ismeasured using a reflectometer utilizing a tungsten lamp at a filamenttemperature of substrates, the reflective coatings that are thusproduced have not been uniform. The manufacturing process itself istedious and time-consuming inasmuch as it requires multiple small glasssubstrates to be laid by hand upon a carrier that passes through amagnetron sputtering apparatus and requires each of the resultingindividual mirror pieces to be removed by hand from the carrier sheetonce the sputtering operation is complete.

To avoid these problems, it would be desirable to first sputter deposita reflective coating on a flat glass sheet or other substrate to form amirror, and then bend and cut the mirror as desired. However, a problemarises when flat glass sheets are coated with the usual reflecting layerusing chromium, for example, as the reflective metal, and then areheat-bent. Once the coated sheets are heated to a temperature sufficientto enable permanent deformation that is, plastic flow--of the substrateto occur (approximately 1110-1130° F. for glass), and the glass is bent,the coatings tend to develop defects which may be referred to as pits.The pits appear as visually detectable small, circular defects havinglittle reflectance. The reason for the development of pitting is notfully understood, but is believed to be a function of stresses developedduring the bending operation in one or more of the reflective sputterdeposited films forming the reflective layer.

SUMMARY OF THE INVENTION

The present invention relates to a heat-formable mirror that is capableof being configured at elevated temperatures into a curved mirrorwithout significant damage to the reflective coating. The reflectivecoating comprises three layers. The first layer is a sputter-depositedbase layer comprising a layer formed of a film of silicon or a layerformed of silicon and stainless steel films with the silicon film nearerthe substrate than the stainless steel film. The second layer is areflective layer that is positioned further from the substrate than thebase layer. It is formed by sputter deposition of a reflective metallicfilm such as niobium, titanium 2854° K. at an angle of incidence of 25°±5° utilizing a detector cell approximately duplicating the human eye(CIE standard photopic curve) and an integrating sphere. In addition togood optical properties for a mirror product, the film stack should bephysically and chemically durable in both the flat and bent states.

DESCRIPTION OF THE DRAWING

FIG. 1 is a broken-away, cross-sectional schematic view of aheat-formable mirror of the invention;

FIG. 2 is a broken-away, schematic, cross-sectional view of anotherembodiment of the invention; and

FIG. 3 is a schematic view showing the use of a heat-bending apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a heat-formable mirror 10 of the invention in which theflat, sheet-like substrate 12 is glass. Glass is the preferredsubstrate, but other substrates that are capable of plastic flow whenheated, such as plastic (e.g., poly (methylmethacrylate)), and metals(eg., aluminum) may also be used. Sputter-deposited on the flat surface20 of the substrate in sequence is a base layer 14, a reflective layer16 and a protective layer 18, to form a reflective coating. In theembodiment of FIG. 1, the base layer 14 is sputter coated directly upona flat surface 20 of the substrate. The base layer comprises a film 22of silicon that is sputter deposited upon the glass surface using amagnetron sputtering technique of the type described in Chapin, U.S.Pat. No. 4,166,018, the teachings of which are incorporated herein byreference. "Sputter deposition", "sputter deposited", and the like areused herein to refer to coatings of the type produced by magnetronsputtering. Magnetron sputtering techniques are well known in the art.

The silicon film 22, it will be understood, may be deposited from asilicon target in an argon atmosphere at about 3 mT. The silicon film 22can vary substantially in thickness, but desirably is between about 300Å and about 1500 Å in thickness. Silicon films 400 Å in thickness havegiven good results. Although we do not wish to be bound to the followingexplanation, it appears that the silicon layer tends to reduce thephysical stresses that are set up in the reflective coating when theheat-formable mirror is bent. However, the use of thicker silicon filmsmay lead to reductions in reflectivity of the bent mirror, and hencesome care must be exercised in controlling the silicon film thickness.

As shown in FIG. 1, the base layer 14 includes a stainless steel film 24deposited over the silicon film 22. The stainless steel film 24 servesto reduce transmissivity of the reflective coating and increasesreflectivity. The thickness of the stainless steel layer 24 may varyconsiderably, but preferably is in the range of about 50 Å to about 250Å with the best results being obtained when the stainless steel layer isof a thickness not exceeding about 225 Å. An additional purpose of thestainless steel film 24 is to inhibit reaction between the underlyingsilicon film 22 and the reflective metal film 16 which is subsequentlyapplied. For example, in the absence of the stainless steel layer 24,reaction between the silicon film 22 and a titanium reflective film 16can result in the formation of titanium silicide, which may reducereflectivity of the reflective coating. When a film of niobium isemployed as the reflective layer, the stainless steel film may beomitted.

Referring again to FIG. 1, a reflective layer 16 is provided over thebase layer 14. Layer 16 desirably is formed through sputter depositionof a reflective metallic film; niobium, titanium and aluminum metalstypify the reflective metal that can be used for layer 16, with niobiumbeing the preferred metal and titanium the next preferred. The thicknessof the reflective layer 16 should be such as to provide the mirror witha reflectance (hemispherical reflectance as measured using areflectometer and integrating sphere over the wavelength range of 200 to2600 nm) of at least 50% and a transmissivity not greater than about4.0%. Reflectance occurs primarily from the outer surface 26 of thelayer 16. As noted above, the underlying stainless steel film 24, whenpresent, contributes to reflectivity. Preferably, the reflective layer16 is of sputter deposited niobium metal having a thickness in the rangeof 200 Å to 500 Å, with a thickness of about 360 Å giving good results.Titanium metal films may have thicknesses in the range of 100 Å-300 Åwith a thickness of about 200 Å giving good results.

Shown at 18 in FIG. 1 is a protective layer which desirably is sputterdeposited directly upon the metallic reflective layer 16, the protectivelayer being of a physically and chemically durable material thatinhibits oxygen permeation to the underlying metal layer or layersduring bending. The protective layer desirably inhibits oxygenpermeability sufficiently to prevent reflectivity from being reduced toless than 50% during heat bending. Films of silicon nitride or aluminumoxide may be employed as or in the protective layer, and it iscontemplated that films of silicon dioxide also could be employed.Sputter deposited silicon nitride films are preferred, and may range inthickness from about 50 Å to about 150 Å with thicknesses in the rangeof about 100 Å being preferred.

Referring now to FIG. 2, a reflective coating 30 is depicted upon theflat surface 20 of a glass sheet 12, the layers of the reflectivecoating being identical to those of FIG. 1 except that the stainlesssteel layer 24 of FIG. 1 is omitted. That is, the base layer 14 of FIG.2 is a film of sputter deposited silicon, the reflective layer 16 is asputter deposited film of a reflective metal such as titanium oraluminum, and the protective layer 18 comprises a film of siliconnitride or aluminum oxide.

It will be understood that other and further layers of sputteredmaterials may be positioned between or on either side of the base layer,the reflective layer and the protective layer, provided such additionallayers do not contribute to objectionable pitting or other failure ofthe reflective coating when the mirror is subject to heat forming. Forexample, stainless steel may be added at the substrate to further reducetransmissivity. Thin aluminum or silver films may be added above orbelow the reflective layer for the purpose of increasing reflectivity.Moreover, thin metal oxide films such as oxides of silicon and titaniumcan be positioned beneath the base layer to improve resistance to theformation of pinholes. Preferably, the respective base, reflective andprotective layers are contiguous, that is, they are formed withoutintermediate layers between them. Thus, the base layer 14 preferably isformed directly upon the flat surface 20 of a glass or other substrate,and to the extent that the base layer 14 is formed of silicon andstainless steel films 22, 24, the latter films desirably are formed withthe stainless steel film 24 directly sputter deposited upon the siliconfilm 22. The reflective metal film 16 desirably is formed by sputterdeposition of niobium, titanium or aluminum directly upon the exposedsurface of the base layer 14. The protective layer 18, in similarfashion, desirably is sputter deposited directly upon the reflectivelayer 16. In a most preferred embodiment, the base layer 14 is silicon,the reflective layer is a film of niobium contiguous to the silverlayer, and the protective layer is a film of silicon nitride contiguousto the niobium film. Niobium is particularly preferred as the reflectivefilm in comparison to titanium or other reflective metals inasmuch asniobium appears to enable the resulting mirror to reside for a longerperiod of time at the bending temperature without damage to the physicalproperties of the mirror.

FIG. 3 depicts a heated mold useful in the heat-formation of curvedglass sheets. Molds of this type are commonly used for this purpose inthe formation of, for example, curved automobile windshields and curvedglass sheets that are subsequently to be provided with a mirroredsurface for use as motor vehicle rearview mirrors and the like. The moldconsists of a female part 40 having a concave upper surface 42, and amale portion 44 having a downwardly facing convex surface 46. In use,the mold portions are heated to the softening temperature of glass, anda heat bendable mirror such as that described in connection with FIG. 1is placed upon the surface of the female member with its reflectivecoating 28 facing downwardly. As the flat glass sheet is heated to itssoftening point, it sags downwardly into conformation with the uppersurface 42 of the mold. The male mold portion 44 is then urgeddownwardly against the other surface of the glass sheet and serves toensure smooth conformation of the glass sheet against the surface 42.Once bending has been completed, the molds are cooled below the glasstransition point of the mirror 10, the mold parts are separated and thebent mirror is removed. The mold operating temperatures commonly are inthe range of 1110-1130° F.

The reflective coatings of mirrors of the invention, before and afterbending, should demonstrate substantial durability. That is, thecoatings should exhibit resistance to abrasion, w to heat and coldextremes, to humidity, and to solvents such as alcohols and salt spray.Resistance to abrasion may be measured by sliding an ordinary pencileraser (Blaisdell® #536T or equivalent), weighted with a 1 kg load, backand forth over a methanol-cleaned coated surface for 100 cycles. Topass, the coating should not display significant film loss or loss ofreflectivity. Adherence of the sputtered-on film stack to the substratecan be tested by attempting to pull off the coating with apressure-sensitive adhesive tape, as described in MILC-48497A.Resistance to alcohol may be tested by rubbing an area on the coatingwith an isopropanol-soaked clean cloth under hand pressure. A salt spraytest is described in ASTM B-117, and is continued for 240 hours. To testfor resistance to humidity, a coated specimen is exposed for 500 hoursin a humidity chamber maintained at 45° C. and 98%-100% relativehumidity. After each of the tests described above, the tested coatingsare visually examined to detect any defects.

EXAMPLE 1

Using a commercial magnetron sputtering line, the upper, cleaned surfaceof flat glass sheets were exposed to sputtering from various targets ina series of successive zones, the speed of travel of the glass sheetsand the electric power delivered to the various magnetron sputteringunits being such as to provide the desired thicknesses of the varioussputtered films. Three of the zones initially encountered by the glasssheets were provided with silicon targets and an argon atmosphere,resulting in the sputter deposition of a silicon film having a thicknessof about 400 Å. The sheets then passed into a zone in which stainlesssteel was sputtered from a stainless steel target to a thickness ofapproximately 175 Å. Following the stainless steel zone, the glasssheets passed through a zone having a titanium target in an argonatmosphere, and titanium metal was sputtered onto the stainless steelsurface to a thickness of about 200 Å. Finally, after exiting thetitanium zone, the glass sheets passed into two sequential zones havingsilicon targets in a nitrogen atmosphere, and silicon nitride wassputter deposited to a final thickness of about 100 Å. The resultingheat-formable mirror was measured for transmission, reflectance andcolor properties and was then subjected to the bending proceduredescribed above at a temperature of approximately 1130° F. Upon removalof the resulting curved mirror from the mold, the mirror was examinedfor coating defects and was also subjected to reflectance,transmissivity, color and durability testing. No haze or other physicaldefect was observed. Reflectance before and after bending was 57%,transmittance of the bent mirror was 2.2%, and the reflective colorcoordinates of the bent film (Hunter L,a,b System, Illuminant D 65) werea=1.22 and b=5.80. Analysis of the finished product indicated somediffusion of iron and chromium from the stainless steel layer into thesilicon layer to form suicides, without harmful effect upon the mirror.

EXAMPLE 2

A heat-formable mirror was formed in a manner substantially identical tothat of Example 1 except that the stainless steel sputtering zone wasomitted. The reflective coating of the resulting mirror thus consistedof a base layer consisting of a silicon film having a thickness of about960 Å, a reflective layer of titanium metal having a thickness of about125 Å, and a protective layer of silicon nitride at a thickness ofapproximately 100 Å. The mirror was heat formed by bending as describedabove in connection with Example 1, and reflectance, transmission andcolor properties were measured before and after the bending procedure.The bent mirror was also inspected for pitting and other defects. Novisual defects or haze was noted either before or after bending.Reflectivity of 50% was obtained before and after bending, andtransmissivity after bending was measured as 3.8%. The bent productexhibited reflected color coordinates of a=-0.45, b=2.38.

To see what chemical changes may have occurred during the heat-formingprocess, the reflective coating of the bent mirror resulting fromExample 2 was subjected to analysis using Auger electron spectroscopy(AES). It was found that the silicon film had reacted with thecontiguous titanium metal film to yield a layer of titanium silicide andcaused a reduction in reflectivity of the reflective coating. Onepurpose of the stainless steel film employed in the reflective coatingshown in FIG. 1 is to serve as a barrier between the silicon andtitanium films to inhibit reaction between them. Notwithstanding thereaction between the silicon and titanium films, the curved mirrorresulting from Example 2 was found to be substantially free of defectsand remained highly reflective.

EXAMPLE 3

A heat-formable mirror was formed by sputter depositing films upon aglass substrate in the manner described above in Example 1. Upon theglass surface was sputtered a first film of silicon at a thickness of700 Å, followed by a second film of niobium at a thickness of 400 Å anda third and last film of silicon nitride at a thickness of. 150 Å. Themirror was heat formed by bending as described above in connection withExample 1, and reflectance, transmission and color properties weremeasured before and after the bending procedure. No visual defects orhaze was noted either before or after bending. Reflectivity before andafter bending was 51.2% and 51.8%, respectively, and transmissivity bothbefore and after bending was 0.4%. The product exhibited reflected colorcoordinates of a=1.5, b=7.0 before bending and a=-0.1, b=3.6 afterbending. The use of niobium as the reflective film permits prolonged(e.g., 20 minutes or longer) bending procedures to be tolerated.

While a preferred embodiment of the present invention has beendescribed, it should be understood that various changes, adaptations andmodifications may be made therein without departing from the spirit ofthe invention and the scope of the appended claims.

What is claimed is:
 1. A heat-formable mirror comprising a flat glasssubstrate and a multi-layer reflective coating formed on one surface ofthe substrate, the mirror being formable at the softening temperature ofthe glass substrate without significant physical damage to thereflective coating, the reflective coating comprising a silicon film, areflective layer comprising a niobium film positioned further from thesubstrate than the silicon film, and a protective layer comprising asilicon nitride film positioned further from the substrate than thenibioum film, the respective films having thicknesses providing thereflective coating with a transmissivity of not greater than about 4%and a reflectance not less than about 50%.
 2. The mirror of claim 1,including, between said silicon film and said reflective layer, asputter-deposited stainless-steel film having a thickness of 50-250 Å.3. A heat-formable mirror comprising a flat glass substrate and amulti-layer reflective coating formed on one surface of the substrate,the mirror being formable at the softening temperature of the glasssubstrate without significant physical damage to the reflective coating,the reflective coating comprising, from the glass substrate outwardly, asilicon film, a reflective film of niobium metal contiguous to thesilicon film, and a silicon nitride film contiguous to the reflectivefilm, the respective films having thicknesses providing the reflectivecoating with a transmissivity of not greater than about 4% and areflectance not less than about 50%.
 4. The heat-formable mirror ofclaim 3 wherein the thickness of the niobium film is in the range of 200Å to 500 Å.
 5. A method of manufacturing a curved mirror, comprising thesteps of providing a mirror according to any one of claims 1-4, heatingthe mirror to a temperature at which the substrate is capable of plasticdeformation, bending the mirror at that temperature into a desiredcurved conformation, and cooling the mirror while maintaining saidcurved conformation.
 6. A curved mirror produced by providing a flatmirror according to any one of claims 1-4, heating said flat mirror to atemperature at which the substrate is capable of plastic deformation,bending said flat mirror at said temperature into a desired curvedconformation to produce a curved mirror, and cooling said curved mirrorwhile maintaining said curved conformation.